Premature and sick newborns frequently are exposed to high concentrations of oxygen for prolonged periods to compensate for inadequate pulmonary gas exchange. The concerns associated with the potential hazards o hyperoxic exposure are magnified in these newborns because their normal antioxidant defense functions are not fully developed. However, the activities of heme oxygenase are higher in the perinatal period than in adults, making the potential contributions of this enzyme to antioxidant defense functions of considerable interest. Heme oxygenase (HO) is the rate-limiting enzyme in the oxidation of heme to biliverdin, which is converted subsequently to bilirubin. Of the two commonly studied isoforms of HO, one is highly inducible (HO-1) and one is constitutive and has not been reported to be inducible (HO-2). The inducibility of HO-1 by a number o stimuli that generate oxidative stresses, the antioxidant activity of the product bilirubin, and the antioxidant protection provided by removal of heme and upregulation of ferritin, suggest that heme oxygenase contributes significantly to antioxidant defense functions. The greater activities of HO observed in the perinatal animals than in adults further suggest a potentially greater role in antioxidant defense functions in newborns. However, relatively little is known about the inducibility of HO-1 in the perinatal period, and important fundamental questions remain regarding the specific molecular mechanisms whereby HO-1 activity is regulated. The purpose of the studies described in the present proposal is to determine the mechanisms involved in regulation of heme oxygenase-I activities in fetal and newborn rat liver and lung. The effects of alterations in glutathione status and oxidant stresses on HO-1 induction will be determined, as will the correlate regulation of ferritin. The contributions of HO activities to antioxidant defense functions in these animals will be examined, initially by pharmacological manipulations of activities and determinations of resistance to oxidants such as hyperoxia and redox- cycling agents such as diquat and paraquat. Further studies are designed to define the molecular mechanisms responsible for regulation of HO expression and activity. Generation of a transgenic mouse in which the HO-I gene is overexpressed in the lungs will be pursued and should provide unique opportunities to test hypotheses regarding the antioxidative contributions of HO activity. The results of the studies described in the present application are intended to provide a rational basis for the design of therapeutic strategies in which the pharmacologic and/or genetic manipulation of heme oxygenase might be used to improve antioxidant defense functions in premature and sick newborns.